What is the importance of Danforth's statement, "A person is either with this court or he must be counted against it" in "The Crucible"?
This statement is directed primarily at Reverend Hale, the expert on witches who came to Salem to ferret out the truth of the matter. When Hale begins to see that there are no witches in Salem...only the "witchcraft" of jealousy and greed which turns neighbors against one another, he begins to question to court. He asks point-blank, "Is anyone innocent?" The court is infuriated at the implications, and even though Danforth all but admits there may be fault with some of their conclusions, he follows up with the fact that no decisions of the court can be reversed since that would call into question the validity of other decisions. It was simply too late to call it quits and allow the "witches" to go free.
This is a hugely important idea in The Crucible. Danforth is saying that those who do not agree with the court's decisions are to be punished similarly to those fighting directly against it.
Think of it this way. Say you were against the death penalty in your home state, and someone was just sentenced to death by the state supreme court. In the world of The Crucible and Danforth, just the fact that you disagree with the system of capital punishment is enough to get you yourself put in the electric chair.
In the Crucible, those who spoke out against the girls and the court's decisions were punished, even if they weren't being accused of witchcraft. (I believe Giles Corey is one who falls under this category.) The obvious problem with this system is that it is nearly impossible for the truth to be brought to light if the truth contradicts the court's current beliefs. Why would anyone speak the truth when they are going to be punished and considered immoral and heretical because of it? Therefore, it is likely that the truth will be kept supressed for a good long time, as was the case during the Salem witch trials.